Touch switch for electrical appliances and electrical appliance provided with such switch

ABSTRACT

A touch switch for an electrical appliance having a capacitive flat electrode, a light source, a transparent cover and a printed circuit board connected to the electrode is disclosed. The light source is mounted on a face of the printed circuit board opposite the transparent cover and the printed circuit board presents a cut-out for allowing light transmission from the light source towards the transparent cover.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/247,407, filed Oct. 8, 2008 which is incorporated herein by referencein its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a touch switch for an electricalappliance to be used on control panels thereof. The invention alsorefers to a capacitive touch switch comprising a capacitive flat sensorsurface, a light source, a transparent cover provided over said touchswitch and a printed circuit board connected to the capacitive sensorelement.

2. Description of the Related Art

With the term “switch” we mean every kind of touch sensors which usuallyreplace the traditional buttons in modern flush user interfaces.

Visual feedback activation of a touch switch control is simply providedby a light source, for example a light emitting diode (LED). Usuallyseveral touch switches are arranged, together with control boardthereof, in a control panel.

To avoid optical interference between adjacent touch switches, lightflux shall be conveyed from a light source to the related touchsensitive switch surfaces.

Opaque walls or light guide means are normally provided for this object.The use of opaque walls increases physical dimensions and renders themounting quite complex.

Light guides can be used as mechanical support for the electrodes.Electrodes can also be made of transparent conductive material (such asindium tin oxide), located between the light guide and the lower surfaceof the transparent cover under which the touch switch is placed, with anincrease of the overall cost of the control panel.

On top of the above illumination problems, there is the need of assuringa good mechanical connection between the touch switch and thetransparent cover (usually a glass or plastic plate) under which theswitch is placed. Up to now, for assuring such mechanical contactsprings or conductive rubber elements have been used, which may increasethe overall cost and complexity of the single touch switch, particularlyas far as the need of assuring a good electrical contact of the springsis concerned. Moreover, technical solutions are known (for instance fromUS-A-2006/0243575) in which a support member, carrying electroniccomponents, is separated from the transparent cover by a series ofopaque septum-like partitions for creating physically delimitedillumination channels. Even if such solution does not require lightguides (light emitting diodes are placed on the support memberdirectly), nevertheless it is quite bulky and of complex manufacturing.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to propose a differentstructural arrangement of a touch switch in order to allow a reductionof physical dimensions of the switch, easy mounting process, andimprovement on versatility for different symbols as painted objects.

Another aspect of the present invention is to provide a touch switchthat has an improved sensibility.

A further aspect of the present invention is to provide a capacitivetouch switch without an external icon serigraphy and which, when notactivated or not in use (when the control panel is off) is not visibleby the user.

A further aspect of the present invention is to provide a touch switchthat can give a direct feedback to the user when a command has been dulyreceived.

A further aspect of the present invention is to provide a touch switchthat is substantially invisible by the user, its presence being detectedonly when the user's finger touches an enabling switch.

The disclosure also discloses a touch switch of the above type in whichthe light source includes an illuminated display.

A capacitive touch sensor of this specific type is disclosed by U.S.Pat. No. 6,403,904 in which a housing of a seven-segment display issoldered to with soldering pins to the printed circuit board detachedfrom the transparent cover. The capacitive flat sensor surface incontact with the transparent cover has a frame-shaped flat portionaround the seven-display housing and is connected to the printed circuitboard by means of standing legs. This technical solution is quite bulky,due to the thickness of the standard seven-segment display package.Moreover it needs a precise mounting for assuring a correct contact ofthe capacitive flat sensor surface against the transparent cover. Inorder to overcome this problem, known touch switches make use ofactuators or conductive elements (springs, sponges, rubbers, etc. . . .) for the electrical contact and the mechanical arrangement betweentouch area and touch sensor.

Another aspect of the above known solution is that the capacitivesurface is obtained starting from a metal sheet from which are punchedor worked the central cut-out and notches, which leads to an increase ofthe overall cost of the touch switch.

Another solution could use a printed circuit board (PCB) having a biggerthickness, for example 4 mm, to avoid the use of actuators or conductiveelements. The use of such thick PCB increases the overall cost of thetouch switch, and moreover it can create problem of differential thermalexpansion particularly when the touch switch is used in cookingappliances (cooking hobs, ovens) where the temperature of the switch canreach values closer to 100° C. or above.

It is a further aspect of the present invention to provide a capacitivetouch switch with an illuminated display that does not present the aboveproblems and that has a reduced cost.

According to the invention, the above aspects are obtained to thefeatures listed in the appended claims.

According to the present invention, no leg is needed to cover the gapbetween touch area and sensor surface due to light source or displaypackage.

Moreover there is no need to use PCB having a big thickness, for example4 mm or above, or multilayered PCB to cover gap between touch area andtouching sensor due to display package.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of a touch switch according to thepresent invention will become clear from the detailed descriptionprovided as a non limiting example, with reference to the attacheddrawing in which:

FIG. 1 shows a cross section of a touch switch according to a firstembodiment of the present invention;

FIG. 2 is a top view of a component of the touch switch of FIG. 1;

FIG. 3 is a view from the bottom of another component (PCB) of the touchswitch of FIG. 1;

FIG. 4 shows a cross section of a touch switch according to a secondembodiment of the present invention;

FIG. 5 shows a cross section of a touch switch according to a thirdembodiment of the present invention;

FIG. 6 is a perspective view of a seven-segment display used in theembodiment of FIG. 5,

FIG. 7 shows a cross section of a touch switch according to a fourthembodiment of the present invention; and

FIG. 8 shows a cross section of a touch switch according to a fifthembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings, a touch switch presents a non-conductivetransparent cover 3, for instance of glass or polymeric material. On thetransparent cover 3 there is at least one touch sensitive area 2 adaptedto be touched by the user's finger 1.

On the lower face of the transparent cover 3, a single side printedcircuit board (PCB) 5 is attached by means of a non-conductivetransparent adhesive 4 (FIG. 5). The PCB 5 is provided with a cut-out10.

With reference to FIG. 1, on the side of the PCB 5 opposite thetransparent cover 3 (i.e. the side with the metallic tracks), there isprovided a plastic light guide 12 in the form of a planar sheet providedwith a protrusion 12 a located in the cut-out 10 of the PCB. The lightguide 12 is fixed to the PCB 5 by means of adhesive or the like, andadjacent the protrusion 12 a the light guide 12 presents a seat 12 b inwhich a LED 14, electrically connected to and supported by the PCB 5, isplaced. In the area surrounding the cut-out 10, the PCB 5 presents anannular track 17 which acts as a touch electrode and is interrupted incorrespondence of the LED 14 (FIG. 3). Between the transparent cover 3and the PCB 5 a polymeric film 16 is interposed, that has the functionof a light filter. The film 16 is substantially opaque and presentscircular transparent zones 16 a where icons K are obtained by printedserigraphy. When the LED 14 is switched on, light is guided in theprotrusion 12 a, possibly with the help of reflective portions R insidethe light guide 12. Therefore light emerges from the protrusion 12 a andis filtered by the film 16, displaying the respective icon K as shown inFIG. 2. The level of illumination may be linked to the electrical stateof the touch switch. For instance, when the appliance is switched on allthe icons K can be illuminated at a first identical level in order toallow the user choosing among them. When the user touches a switch, thelevel of illumination of that single switch is automatically increasedto a second higher level in order to give the user an immediatefeedback. The use of the film 16 improves the flexibility of use of theuser interface. For example, by just changing this layer it is possibleto easily customize the user interface for different countries andlanguages. When a big light diffusion effect is requested, the film 16can be thicker (few millimetres) with “ad hoc” treated surfaces,otherwise this film can be very thin or it can be removed when theserigraphy is printed directly on the transparent cover 3.

Even if in the example of FIGS. 1 to 3 the touch electrode 17 isrealized as a circular crown with a hole in the centre for the lightguide, other shapes can be easily adopted.

The embodiment according to FIG. 4 differs from the previous one becauseno light guide is needed. In FIG. 4 parts which are identical or similarto the parts of the previous embodiment are indicated with the samereference numerals. On a face of the PCB 5 opposite the transparentcover 3 a body package 9 of a reversed mounted LED is soldered aroundthe cut-out 10 by means of metal pins. Also in this case the touchelectrode 17 is realized as a circular crown.

In the embodiment shown in FIGS. 5 and 6, the body package around thecut-out 10 is a seven-segment display 9′ which is soldered by means ofmetal pins 8. On the same surface of the PCB 5 there are provided twoelectrodes 6 that, for examples, can be used for increasing (+) ordecreasing (−) a certain parameter. The mounting of the seven-segmentdisplay 9′ is very easy. A low cost single side PCB 5 with standardthickness as FR4 or CEM1 type can be used. No conductive elements likespring or sponges or conductive rubber have to be used. An air gapbetween the seven-segment display 9′ and transparent cover assuresbetter performances in hot applications like cooktops or oven. Moreover,when the display is switched off, it is not seen by the user (darkeffect on surface when off). A further advantage of using a display asthe one shown in FIG. 6 relates to the possibility of using a displaywhose thickness may be different from the thickness of the PCB. As amatter of fact in the standard known solution the thickness of thedisplay must be identical to the thickness of the PCB, and the solderingpart of the display must be flush with the surface of the PCB. This isno longer needed with a display according to the present invention.

Another embodiment is shown in FIG. 7, which is quite similar to theembodiment according to FIGS. 1 to 3 but it can be used also when thetransparent cover 3 is not flat. In this embodiment the protrusion 12 aof the light guide 12 is not substantially flush with the PCB 5, ratherit goes beyond the plane of the PCB towards the transparent cover 3that, in this case, is detached from the PCB. For assuring a properworking of the touch switch, a metal spring 20 is interposed between thePCB 5 and the film 16 carrying the icons serigraphy K and in contactwith the transparent cover 3. The spring 20 has an enlarged end 20 asandwiched between the PCB (on the circular track 17) and the lightguide 12.

The embodiment according to FIG. 7 provides the following functionalityand advantages. The more large part 20 a of the spring 20 compressedbetween the PCB and the plastic light guide or support provides a goodelectrical contact between the touch electrodes 17 (contacts) and thespring. This arrangement avoids the cost for a double side PCB and for awelding process of the spring. To avoid the welding process allows usinga cheaper material for the spring 20 (no tinned steel is required). Theother end of the spring 20 is pressed on upper layers (user interfacesurface and light filter and serigraphy), providing the electricalcontinuity of the touch sensor closed to the user interface surface(avoiding the air gap between touch sensor and user interface surface).

The touch performance is improved if more spring coils when pressed areclosed to the user interface surface, because this improves the surfaceof the capacitor plate of the sensor.

A further embodiment similar to the previous one is shown in FIG. 8where the spring 20′ does not have any enlarged end and it is interposedbetween the PCB 5 and the transparent cover 3 (with the optionalinterposition of the plastic film 16 with serigraph). In this solutionthe touch sensors/electrodes 17′ are realized on PCB in the oppositeside where LED 14 is placed.

1. A capacitive touch switch for an appliance comprising: anon-conductive transparent cover; a printed circuit board below thenon-conductive transparent cover, said printed circuit board including afirst side, an opposing second side and a cut-out portion; an electrodemounted on the second side of the printed circuit board adjacent thecut-out portion and opposite the non-conductive transparent cover; and alight source mounted across the cut-out portion with an air gap beingmaintained between the light source and the transparent cover, and thelight emitted by the light source being directed through the cut-outportion of the printed circuit board towards the non-conductivetransparent cover.
 2. The capacitive touch switch of claim 1, whereinthe light source is mounted on the second side of the printed circuitboard.
 3. The capacitive touch switch of claim 1, further comprising: acapacitive flat sensor portion provided on the non-conductivetransparent cover.
 4. The capacitive touch switch of claim 1, furthercomprising: a second electrode mounted on the second side of the printedcircuit board adjacent the cut-out portion and opposite thenon-conductive transparent cover.
 5. The capacitive touch switch ofclaim 1, wherein the light source is a seven-segment display.
 6. Thecapacitive touch switch of claim 1, wherein the light source is areverse-mounted LED.
 7. The capacitive touch switch of claim 1, whereinthe printed circuit board is fixed to the non-conductive transparentcover by a non-conductive adhesive.
 8. The capacitive touch switch ofclaim 1, further comprising: an auxiliary layer interposed between thenon-conductive transparent cover and the printed circuit board, theauxiliary layer having a transparent zone through which the light fromthe light source is directed.
 9. The capacitive touch switch of claim 8,further comprising: an opaque icon provided at the transparent zone ofthe auxiliary layer.
 10. The capacitive touch switch of claim 8, whereinthe auxiliary layer is a light filtering polymeric film.
 11. Thecapacitive touch switch of claim 1, further comprising: metal pinsfixing the light source to the printed circuit board.